A conventional MIMO technology will hereinafter be described in detail.
In brief, the MIMO technology is an abbreviation of the Multi-Input Multi-Output technology. The MIMO technology uses multiple transmission (Tx) antennas and multiple reception (Rx) antennas to improve the efficiency of Tx/Rx data, whereas a conventional art has generally used a single transmission (Tx) antenna and a single reception (Rx) antenna. In other words, the MIMO technology allows a transmitter or receiver of a wireless communication system to use multiple antennas (hereinafter referred to as a multi-antenna), so that the capacity or performance can be improved. For the convenience of description, the term “MIMO” can also be considered to be a multi-antenna technology.
In more detail, the MIMO technology is not dependent on a single antenna path to receive a single total message, collects a plurality of data pieces received via several antennas, and completes total data. As a result, the MIMO technology can increase a data transmission rate at a give channel condition, or can increase a system performance at a specific data transmission rate.
The next-generation mobile communication technology requires a data transmission rate higher than that of a conventional mobile communication technology, so that it is expected that the effective MIMO technology is requisite for the next-generation mobile communication technology. Under this situation, the MIMO communication technology is the next-generation mobile communication technology capable of being applied to mobile communication terminals or base stations, and can extend the range of a data communication range, so that it can overcome the limited amount of transfer data of other mobile communication systems due to a variety of limited situations.
Among a variety of technologies capable of improving the transmission efficiency of data, the MIMO technology can greatly increase an amount of communication capacity and Tx/Rx performances without allocating additional frequencies or increasing an additional power. Due to these technical advantages, most companies or developers have intensively paid attention to this MIMO technology.
FIG. 1 is a block diagram illustrating a conventional MIMO communication system.
Referring to FIG. 1, if the number of transmission (Tx) antennas increases to NT, and at the same time the number of reception (Rx) antennas increases to NR, a theoretical channel capacity of the MIMO communication system increases in proportion to the number of antennas, so that a transmission rate and a frequency efficiency can greatly increase.
In this case, the transmission rate acquired by the increasing channel capacity is equal to the multiplication of a maximum transmission rate (Ro) acquired when a single antenna is used and a rate increment (Ri), and can theoretically increase. The rate increment (Ri) can be represented by the following equation 1:Ri=min(NT,NR)  [Equation 1]
For example, provided that the MIMO system uses four Tx antenna and four Rx antennas, this MIMO system can theoretically acquire a high transmission rate which is four times higher than that of a single antenna system.
After the above-mentioned theoretical capacity increase of the MIMO system has been demonstrated in the mid-1990s, many developers are conducting intensive research into a variety of technologies which can substantially increase a data transmission rate using the theoretical capacity increase. Some of them have been reflected in a variety of wireless communication standards, for example, a third-generation mobile communication or a next-generation wireless LAN, etc.
A variety of MIMO-associated technologies have been intensively researched by many companies or developers, for example, research into an information theory associated with a MIMO communication capacity calculation under various channel environments or multiple access environments, research into a wireless channel measurement and modeling of the MIMO system, and research into a space-time signal processing technology.
The above-mentioned MIMO technology can be classified into two types: a spatial diversity scheme and a spatial multiplexing scheme. The spatial diversity scheme increases transmission reliability using symbols passing various channel paths. The spatial multiplexing scheme simultaneously transmits a plurality of data symbols via a plurality of Tx antennas, so that it increases a transmission rate of data. In addition, the combination of the spatial diversity scheme and the spatial multiplexing scheme has also been recently developed to properly acquire unique advantages of the two schemes.
Details of the spatial diversity scheme, the spatial multiplexing scheme, and the combination thereof will hereinafter be described.
Firstly, the spatial diversity scheme will hereinafter be described. By and large, the spatial diversity scheme is divided into two types: a space-time block code scheme and a space-time Trellis code scheme which can simultaneously uses a diversity gain and a coding gain. Generally, a bit error ratio (BER) improvement performance and a code-generation degree of freedom of the space-time Trellis code scheme are superior to those of the space-time block code scheme, whereas the calculation complexity of the space-time block code scheme is higher than that of the space-time Trellis code scheme.
The above-mentioned spatial diversity gain corresponds to the product or multiplication of the number (NT) of Tx antennas and the number (NR) of Rx antennas, as denoted by NT×NR.
Secondly, the spatial multiplexing scheme will hereinafter be described. The spatial multiplexing scheme is adapted to transmit different data streams via individual Tx antennas. In this case, a receiver may unavoidably generate mutual interference between data pieces simultaneously transmitted from a transmitter. The receiver removes this mutual interference from the received data using a proper signal processing technique, so that it can receive the desired data having no interference. In order to remove noise or interference from the received data, a maximum likelihood receiver, a ZF (Zero Forcing) receiver, a MMSE (Minimum Mean Square Error) receiver, a D-BLAST, or a V-BLAST may be used. Specifically, if a transmitter can recognize channel information, a Singular Value Decomposition (SVD) scheme may be used to remove the interference perfectly.
Thirdly, the combination of the spatial diversity scheme and the spatial multiplexing scheme will hereinafter be described. Provided that only a spatial diversity gain is acquired, the performance-improvement gain is gradually saturated in proportion to an increasing diversity order. As a result, a variety of schemes capable of acquiring all the above-mentioned two gains simultaneously while solving the above-mentioned problems have been intensively researched by many companies or developers, for example, a double-STTD scheme and a space-time BICM (STBICM) scheme.
A mathematical modeling of a communication method for use in the above-mentioned MIMO system will hereinafter be described in detail.
Firstly, as can be seen from FIG. 1, it is assumed that NT Tx antennas and NR Rx antennas exist.
In the case of a transmission (Tx) signal, a maximum number of transmission information pieces is NT under the condition that NT Tx antennas are used, so that the Tx signal can be represented by a specific vector shown in the following equation 2:s=[S1,s2, . . . ,sNT]T  [Equation 2]
The individual transmission information pieces (s1, s2, . . . , sNT) may have different transmission powers. In this case, if the individual transmission powers are denoted by (P1, P2, . . . , PNT), transmission information having an adjusted transmission power can be represented by a specific vector shown in the following equation 3:ŝ=[ŝ1,ŝ2, . . . ,ŝNT]T=[P1s1,P2s2, . . . ,PNTsNT]T  [Equation 3]
In Equation 3, using a diagonal matrix of a transmission power P, Ŝ can be represented by the following equation 4:
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Equation          ⁢                                          ⁢          4                ]            
The information vector Ŝ having an adjusted transmission power is multiplied by a weight matrix (W), so that NT transmission (Tx) signals (x1, x2, . . . , xNT) to be actually transmitted are configured. In this case, the weight matrix is adapted to properly distribute Tx information to individual antennas according to Tx-channel situations. The above-mentioned Tx signals (x1, x2, . . . , xNT) can be represented by the following equation 5 using the vector (x):
                    x        =                              [                                                                                x                    1                                                                                                                    x                    2                                                                                                ⋮                                                                                                  x                    i                                                                                                ⋮                                                                                                  x                                          N                      T                                                                                            ]                    =                                                                                   [                                                                                                              w                          11                                                                                                                      w                          12                                                                                            …                                                                                              w                                                      1                            ⁢                                                                                                                  ⁢                                                          N                              T                                                                                                                                                                                                                    w                          21                                                                                                                      w                          22                                                                                            …                                                                                              w                                                      2                            ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            w                                                      i                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                w                                                      i                            ⁢                                                                                                                  ⁢                            2                                                                                                                      …                                                                                              w                                                      iN                            T                                                                                                                                                              ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            w                                                                                    N                              T                                                        ⁢                            1                                                                                                                                                w                                                                                    N                              T                                                        ⁢                            2                                                                                                                      …                                                                                              w                                                                                    N                              T                                                        ⁢                                                          N                              T                                                                                                                                                            ]                                ⁡                                  [                                                                                                                                          s                            ^                                                    1                                                                                                                                                                                          s                            ^                                                    2                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                    j                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                                                N                            T                                                                                                                                ]                                            =                                                W                  ⁢                                                                          ⁢                                      s                    ^                                                  =                WPs                                                                        [                  Equation          ⁢                                          ⁢          5                ]            
In Equation 5, wij is a weight between the i-th Tx antenna and the j-th Tx information, and W is a matrix indicating the weight wij. The matrix W is called a weight matrix or a precoding matrix.
The above-mentioned Tx signal (x) can be considered in different ways according to two cases, i.e., a first case in which the spatial diversity is used and a second case in which the spatial multiplexing is used.
In the case of using the spatial multiplexing, different signals are multiplexed and the multiplexed signals are transmitted to a destination, so that elements of the information vector (s) have different values. Otherwise, in the case of using the spatial diversity, the same signal is repeatedly transmitted via several channel paths, so that elements of the information vector (s) have the same value.
Needless to say, the combination of the spatial multiplexing scheme and the spatial diversity scheme may also be considered. In other words, the same signal is transmitted via three Tx antennas according to the spatial diversity scheme, and the remaining signals are spatially multiplexed and then transmitted to a destination.
Next, if NR Rx antennas are used, Rx signals (y1, y2, . . . , yNR) of individual antennas can be represented by a specific vector (y) shown in the following equation 6:y=[y1,y2, . . . ,yNR]T  [Equation 6]
If a channel modeling is set up in the MIMO communication system, individual channels can be distinguished from each other according to Tx/Rx antenna indexes. A specific channel from a Tx antenna (j) to an Rx antenna (i) is denoted by hij. In this case, it should be noted that the first index of the channel hij indicates an Rx-antenna index and the second means a Tx-antenna index.
Several channels are tied up, so that they are displayed in the form of a vector or matrix. An exemplary vector is as follows.
FIG. 2 shows channels from NT Tx antennas to an Rx antenna (i).
Referring to FIG. 2, the channels from the NT Tx antennas to the Rx antenna (i) can be represented by the following equation 7:hiT=[hi1,hi2, . . . ,hiNT]  [Equation 7]
If all channels from the NT Tx antennas to NR Rx antennas are denoted by the matrix composed of Equation 7, the following equation 8 is acquired:
                    H        =                              [                                                                                h                    1                    T                                                                                                                    h                    2                    T                                                                                                ⋮                                                                                                  h                    i                    T                                                                                                ⋮                                                                                                  h                                          N                      R                                        T                                                                        ]                    =                      [                                                                                h                    11                                                                                        h                    12                                                                    …                                                                      h                                          1                      ⁢                                              N                        T                                                                                                                                                              h                    21                                                                                        h                    22                                                                    …                                                                      h                                          2                      ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          i                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      h                                          iN                      T                                                                                                                    ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                                                  N                        R                                            ⁢                      1                                                                                                            h                                                                  N                        R                                            ⁢                      2                                                                                        …                                                                      h                                                                  N                        R                                            ⁢                                              N                        T                                                                                                                  ]                                              [                  Equation          ⁢                                          ⁢          8                ]            
An Additive White Gaussian Noise (AWGN) is added to an actual channel which has passed the channel matrix H shown in Equation 8. The AWGN (n1, n2, . . . , nNR) added to each of NR Rx antennas can be represented by a specific vector shown in the following equation 9:n=[n1,n2, . . . ,nNR]T  [Equation 9]
By the above-mentioned modeling method of the Tx signal, Rx signal, and channels including AWGN, each MIMO communication system can be represented by the following equation 10:
                    y        =                              [                                                                                y                    1                                                                                                                    y                    2                                                                                                ⋮                                                                                                  y                    i                                                                                                ⋮                                                                                                  y                                          N                      R                                                                                            ]                    =                                                                                                       [                                                                                                                        h                            11                                                                                                                                h                            12                                                                                                    …                                                                                                      h                                                          1                              ⁢                                                              N                                T                                                                                                                                                                                                                                      h                            21                                                                                                                                h                            22                                                                                                    …                                                                                                      h                                                          2                              ⁢                                                              N                                T                                                                                                                                                                                                          ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  h                                                          i                              ⁢                                                                                                                          ⁢                              1                                                                                                                                                            h                                                          i                              ⁢                                                                                                                          ⁢                              2                                                                                                                                …                                                                                                      h                                                          iN                              T                                                                                                                                                                            ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  h                                                                                          N                                R                                                            ⁢                              1                                                                                                                                                            h                                                                                          N                                R                                                            ⁢                              2                                                                                                                                …                                                                                                      h                                                                                          N                                R                                                            ⁢                                                              N                                T                                                                                                                                                                          ]                                    ⁡                                      [                                                                                                                        x                            1                                                                                                                                                                            x                            2                                                                                                                                                ⋮                                                                                                                                                  x                            j                                                                                                                                                ⋮                                                                                                                                                  x                                                          N                              T                                                                                                                                            ]                                                  +                                  [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                                                    ⋮                                                                                                                                      n                          i                                                                                                                                    ⋮                                                                                                                                      n                                                      N                            R                                                                                                                                ]                                            =                              Hx                +                n                                                                        [                  Equation          ⁢                                          ⁢          10                ]            
The above-mentioned description has disclosed that the MIMO communication system is applied to a single user. However, the MIMO communication system may also be applied to several users, so that it can acquire a multi-user diversity. A detailed description of the multi-user diversity will hereinafter be described.
The fading channel is a major cause of deterioration of a performance of a wireless communication system. A channel gain value is changed according to time, frequency, and space. The lower the channel gain value, the lower the performance. A representative method for solving the above-mentioned fading problem is a diversity. This diversity uses the fact that there is a low probability that all independent channels have low gain values. A variety of diversity methods can be applied to the present invention, and the above-mentioned multi-user diversity is considered to be one of them.
If several users are present in a cell, channel gain values of individual users are stochastically independent of each other, so that the probability that all the users have low gain values is very low. If a Node-B has sufficient transmission (Tx) power and several users are present in a cell, it is preferable that all channels be allocated to a specific user having the highest channel gain value to maximize a total channel capacity according to the information theory. The multi-user diversity can be classified into three kinds of diversities, i.e., a temporal multi-user diversity, a frequency multi-user diversity, and a spatial multi-user diversity.
The temporal multi-user diversity is adapted to allocate a channel to a specific user having the highest gain value when a channel situation changes with time.
The frequency multi-user diversity is adapted to allocate a sub-carrier(s) to a specific user having the highest gain value in each frequency band in a frequency multi-carrier system such as an Orthogonal Frequency Division Multiplexing (OFDM) system.
If a channel situation slowly changes with time in another system which does not use the multi-carrier, the user having the highest channel gain value will monopolize the channel for a long period of time, other users are unable to communicate with each other. In this case, in order to use the multi-user diversity, there is a need to induce the channel to change.
Next, the spatial multi-user diversity uses different channel gain values of users according to space types. An implementation example of the spatial multi-user diversity is a Random BeamForming (RBF) method. This RBF method performs beamforming with a predetermined weight using multiple antennas (i.e., multi-antenna) to induce the change of channel, and uses the above-mentioned spatial multi-user diversity.
The multi-user MIMO scheme which uses the multi-user diversity as the multi-antenna scheme will hereinafter be described in detail.
According to the multi-user multi-antenna scheme, the number of users and the number of antennas of each user can be combined with each other in various ways at transmission/receivers.
The multi-user MIMO scheme is classified into a downlink method (i.e., a forward-link method) and an uplink method (i.e., a reverse-link method), and detailed descriptions of the downlink and uplink methods will hereinafter be described. In this case, the downlink indicates that a signal is transmitted from a Node-B to several user equipments (UEs), and the uplink indicates that several UEs transmit a signal to the Node-B.
The downlink in MIMO can be generally categorized into two kinds of signal reception methods: The first reception method enables a single user (i.e., a single UE) to receive a desired signal via a total of NR antennas, and the second reception method enables each of the NR UEs to receive a desired signal via a single antenna. If required, a combination of the first and second reception methods may also be made available for the present invention. In other words, some UEs may use a single Rx antenna, or some other UEs may use three Rx antennas. It should be noted that a total number of Rx antennas in all combinations is maintained at NR. This case is generally called a MIMO Broadcast Channel (BC) or a Space Division Multiple Access (SDMA).
The uplink in MIMO can be generally classified into two kinds of signal transmission methods: The first transmission method enables a single UE to transmit a desired signal via NT antennas, and the second transmission method enables each of the NT UEs to transmit a desired signal via a single antenna. If required, a combination of the first and second transmission methods may also be made available for the present invention. In other words, some UEs may use a single Tx antenna, or some other UEs may use three Tx antennas. It should be noted that a total number of Tx antennas in all combinations is maintained at NT. This case is generally called a MIMO Multiple Access Channel (MAC).
The uplink and the downlink are symmetrical to each other, so that a method for use in one of them may also be used for the other one.
For the convenience of description and better understanding of the present invention, although the following description will basically describe the MIMO BC, it should be noted that the method of the present invention be also used for the MIMO MAC.
FIG. 3A is a conceptual diagram illustrating a single-user MIMO communication system. FIG. 3B is a conceptual diagram illustrating a multi-user MIMO communication system.
For the convenience of description, FIGS. 3A and 3B assume the use of a downlink.
The single-user MIMO communication system shown in FIG. 3A includes a transmitter (i.e., Node-B) equipped with multiple antennas (i.e., multi-antenna) and a receiver (i.e., UE) equipped with multiple antennas. In this case, if a signal (x) to be transmitted from the transmitter is multiplied by a weight vector (W), and the multiplied resultant signal is transmitted via the multi-antenna, the present invention can acquire a maximum of channel capacity on the assumption that channel information has been correctly recognized.
In the meantime, the multi-user MIMO communication system shown in FIG. 3B includes a plurality of Multiple Input Single Output (MISO) systems, each of which assigns a single antenna to each user. Therefore, the multi-user can maximize the channel capacity using a transmission beamforming in the same manner as in the single-user MIMO communication system. In this case, the multi-user MIMO communication system must consider not only the channel information but also interference of each user, so that it requires a more complicated system than that of the single-user MIMO communication system. Therefore, the multi-user MIMO communication system must select a weight vector to minimize the interference between users in the case of using the transmission beamforming.
The above-mentioned description can be numerically described as follows.
Firstly, the single-user environment, i.e., the single-user MIMO communication system, will hereinafter be described.
Provided that all transmission/receivers have fully recognized all channel information, a singular value decomposition (SVD) H can be represented by the following equation 11:H=UΣVH  [Equation 11]
where “H” is a singular value decomposition, U and V is a unitary matrix, Σ is a diagonal matrix.
In this case, in order to acquire a maximum gain in the light of channel capacity, the diagonal matrix V is selected by the weight matrix W, and UH is multiplied by a reception signal (Y). If the resultant signal of the receiver is denoted by {tilde over (y)}, the following equation 12 is acquired:W=V y=Hx+n=UΣVHx+n=UΣVHWŝ+n=UΣVHVŝ+n=UΣŝ+n {tilde over (y)}=UHy=UHUΣŝ+UHn=Σŝ+ñ=ΣPs+ñ  [Equation 12]
where P is a transmission power matrix. The transmission power matrix P can be determined by a specific algorithm (well known as a water-filling algorithm) for acquiring the channel capacity. This water-filling algorithm is an optimum method for acquiring the channel capacity.
However, in order to perform the water-filling algorithm, all the transmission/receivers must completely know all channel information. Therefore, in order to use the water-filling algorithm under the multi-user environment, each of all users must know not only his or her channel information but also channel information of other users. Due to this problem, in fact, it is almost impossible for the multi-user MIMO communication system to use the above-mentioned water-filling algorithm.
Next, the multi-user MIMO communication system will hereinafter be described.
In this case, a representative optimum method for acquiring the channel capacity is a Dirty Paper Coding (DPC) method, but this DPC method has high complexity. Also, there are other optimum methods for use in the present invention, for example, a Random BeamForming (RBF) and a Zero Forcing BeamForming (ZFBF). The above-mentioned RBF or ZFBF method may have a performance similar to the optimum performance acquired by the DPC method, if the number of users increases in the multi-user environment.
In the meantime, a codeword for use in the MIMO communication system will hereinafter be described.
A general communication system performs coding of transmission information of a transmitter using a forward error correction code, and transmits the coded information, so that an error experienced at a channel can be corrected by a receiver. The receiver demodulates a received (Rx) signal, and performs decoding of forward error correction code on the demodulated signal, so that it recovers the transmission information. By the decoding process, the Rx-signal error caused by the channel is corrected.
Each of all forward error correction codes has a maximum-correctable limitation in a channel error correction. In other words, if a reception (Rx) signal has an error exceeding the limitation of a corresponding forward error correction code, a receiver is unable to decode the Rx signal into information having no error. Therefore, the receiver must determine the presence or absence of an error in the decoded information. In this way, a specialized coding process for performing error detection is required, separately from the forward error correction coding process. Generally, a Cyclic Redundancy Check (CRC) code has been used as an error detection code.
The CRC method is an exemplary coding method for performing the error detection. Generally, the transmission information is coded by the CRC method, and then the forward error correction code is applied to the CRC-coded information. A single unit coded by the CRC and the forward error correction code is generally called a codeword.
In the meantime, if several transmission information units are overlapped and then received, the present invention can expect performance improvement using an interference-cancellation receiver. There are many cases in the above-mentioned case in which several transmission information is overlapped and then received, for example, a case in which the MIMO technology is used, a case in which a multi-user detection technology is used, and a case in which a multi-code technology is used. A brief description of the interference-cancellation structure will be as follows.
According to the interference-cancellation structure, after first information is demodulated/decoded from a total reception signal in which several information is overlapped, information associated with the first information is removed from the total reception signal. A second signal is demodulated/decoded by the resultant signal having no first information removed from the reception signal. A third signal is demodulated/decoded by the resultant signal having no first- and second-information removed from the first reception signal. A fourth signal or other signal after the fourth signal repeats the above-mentioned processes, so that the fourth or other signal is demodulated/decoded. In this way, the above-mentioned method for continuously removing the demodulated/decoded signal from a reception signal to improve a performance of the next demodulating/decoding process is called a Successive Interference Cancellation (SIC) method.
In order to use the above-mentioned interference cancellation method such as the SIC, the demodulated/decoded signal removed from the reception signal must have no error. If any error occurs in the demodulated/decoded signal, an error propagation occurs so that a negative influence continuously affects all the demodulated/decoded signals.
The above-mentioned interference cancellation technology can also be applied to the MIMO technology. If several transmission information pieces are overlapped/transmitted via multiple antennas, the above-mentioned interference cancellation technology is required. In other words, if the spatial multiplexing technology is used, each transmitted information is detected, and at the same time the interference cancellation technology can be used.
However, as described above, in order to minimize the error propagation caused by the interference cancellation, it is preferable that the interference is selectively removed after determining the presence or absence of an error in the demodulated/decoded signal. A representative method for determining the presence or absence of the error in each transmission information is the above-mentioned cyclic redundancy check (CRC) method. A unit of distinctive information processed by the CRC coding is called a codeword. Therefore, a more representative method for using the interference cancellation technology is a specific case in which several transmission information pieces and several codewords are used.
In the meantime, the number of rows and the number of columns of a channel matrix H indicating a channel condition is determined by the number of Tx/Rx antennas. In the channel matrix H, the number of rows is equal to the number (NR) of Rx antennas, and the number of columns is equal to the number (NT) of Tx antennas. Namely, the channel matrix H is denoted by NR×NT matrix.
Generally, a matrix rank is defined by a smaller number between the number of rows and the number of columns, in which the rows and the columns are independent of each other. Therefore, the matrix rank cannot be higher than the number of rows or columns. The rank of the channel matrix H can be represented by the following equation 13:rank(H)≤min(NT,NR)  [Equation 13]
Another definition of the above-mentioned rank can be defined by the number of eigen values other than “0” when the matrix is eigen-value-decomposed. Similarly, if the rank is SVD-processed, the rank may also be defined by the number of singular values other than “0”. Therefore, the physical meaning of the rank in the channel matrix may be considered to be a maximum number of transmission times of a given channel capable of transmitting different information.
For the convenience of description, it is assumed that each of different information pieces transmitted via the MIMO technology is a transmission stream or a stream. This stream may also be called a layer, so that the number of transmission streams cannot be higher than the channel rank equal to the maximum number of transmission times of the channel capable of transmitting different information.
If the channel matrix is H, this channel matrix H can be represented by the following equation 14:# of streams≤rank(H)≤min(NT,NR)  [Equation 14]
where “# of streams” is indicative of the number of streams.
In the meantime, it should be noted that a single stream may be transmitted via one or more antennas.
A method for matching the stream with the antenna can be described according to the MIMO technology types.
In the case where a single stream is transmitted via several antennas, this case may be considered to be the spatial diversity scheme. In the case where several streams are transmitted via several antennas, this case may be considered to be the spatial multiplexing scheme. Needless to say, a hybrid scheme between the spatial diversity scheme and the spatial multiplexing scheme may also be made available.
The relationship between the codeword and the stream in the MIMO communication system will hereinafter be described in detail.
FIG. 4 is a block diagram illustrating the relationship between the codeword and the stream in the MIMO communication system.
A variety of methods for matching the codeword with the stream are made available. A general method from among the various methods generates codeword(s), allows each codeword to enter a codeword-stream mapping module, matches the codeword received from the codeword-stream mapping module with the stream(s), and transmits the stream to the stream-antenna mapping module, so that the stream is transmitted via the Tx antenna.
A part for determining the combination between the codeword and the stream is denoted by a bold solid line in FIG. 4.
Ideally, the relationship between the codeword and the stream can be freely determined. A single codeword may be divided into several streams, so that the divided streams are transmitted to a destination. Several codewords are serially integrated in one stream, so that this stream including the codewords may be transmitted to a destination.
However, the above-mentioned serial-integration of several codewords may be considered to be a kind of predetermined coding process, so that the present invention assumes that a single codeword is matched with one or more streams of a real-meaningful combination. However, provided that several streams are distinguished from each other without departing from the scope or spirit of the present invention, the present invention can also be applied to the distinguished streams.
Therefore, for the convenience of description, the present invention assumes that a single codeword is matched with one or more streams. Therefore, if all information is coded and then transmitted to a destination, the following equation 15 can be acquired:# of codewords≤# of streams  [Equation 15]
where “# of codewords” is the number of codewords, and “# of streams” is the number of streams.
In conclusion, the above-mentioned equations 13 to 15 can be represented by the following equation 16:# of codewords≤# of streams≤rank(H)≤min(NT,NR)  [Equation 16]
By Equation 16, the following fact can be recognized. In other words, if the number of Tx/Rx antennas is limited, a maximum number of streams is also limited. If the number of codewords is limited, a minimum number of streams is also limited.
By the above-mentioned relationship between the codeword and the stream, if the number of antennas is limited, the maximum number of codewords or streams is limited, so that the limited number of codewords can be combined with the limited number of streams.
The above-mentioned combination between the codeword and the stream is required for both an uplink and a downlink.
For example, it is assumed that the MIMO technology is applied to the downlink. In this case, a receiver must correctly be informed of a combination beforehand, which is used for the above-mentioned information transmission from among all combinations between the codeword and the stream, so that the demodulating/decoding process of the information can be correctly performed.
Also, if control information is transmitted to the uplink, a preferred combination from among various combinations between the codeword and the stream must also be recognized by a receiver. In more detail, in order to implement the MIMO technology, a transmitter must recognize channel and status information of a receiver, so that the receiver must notify various control information via the uplink.
For example, the receiver considers a variety of receiver states (e.g., a measured channel or buffer status), and must notify a preferred combination between a codeword and a stream, a channel quality indicator (CQI) corresponding to this preferred combination, and a precoding matrix index (PMI) corresponding to the same. Needless to say, the contents of detailed control information may be differently determined according to the type of a used MIMO technology. However, the above-mentioned fact in which the receiver must inform the uplink of the preferred combination between the codeword and the stream is unchangeable.
For another example, if the MIMO technology is applied to the uplink, only a transmission link is changed to another link differently from the above-mentioned example's description, and the remaining facts other than the change of the transmission link are equal to those of the above-mentioned example, so that all combination between a codeword and a stream, a used combination, and a preferred combination must be notified.
If all the combinations between the codeword and the stream can be indicated by a small number of bits, control information can be more effectively transmitted to a destination. Therefore, there is needed a method for effectively indicating the combination between the codeword and the stream.